Whenever a P-N junction diode is forward biased, the width of the space charge region _________ and the forward current __________ for rise in supply voltage.

Explanation:

Behavior of a P-N Junction Diode under Forward Bias:

Definition: A P-N junction diode is a semiconductor device formed by joining p-type and n-type materials. The region where these two materials meet is called the junction, and this junction is surrounded by a depletion or space charge region. When a P-N junction is forward biased, the external voltage reduces the potential barrier at the junction, enabling charge carriers to flow freely across it.

Working Principle:

Under forward bias, the positive terminal of the voltage source is connected to the p-side (anode), and the negative terminal is connected to the n-side (cathode). This causes the potential barrier at the junction to decrease, allowing holes from the p-side and electrons from the n-side to move towards the junction. As the voltage increases, the flow of charge carriers increases, resulting in an increase in forward current.

Explanation of the Correct Option:

The correct answer is:

Option 3: The width of the space charge region decreases, and the forward current increases for a rise in supply voltage.

When the diode is forward biased, the external voltage reduces the width of the depletion region (space charge region). This occurs because the applied voltage opposes the built-in electric field across the junction, reducing the barrier potential. As the depletion region becomes narrower, more charge carriers (holes and electrons) can cross the junction, leading to an increase in the forward current.

The relationship between the forward current (I_F) and the applied voltage (V_F) is exponential in nature, as described by the diode equation:

I_F = I₀ × (e^qV_F/kT - 1)

Where:

• I₀: Reverse saturation current
• q: Charge of an electron (1.6 × 10^-19 C)
• k: Boltzmann constant (1.38 × 10^-23 J/K)
• T: Temperature in Kelvin

This equation shows that even a small increase in the forward voltage can lead to a significant rise in the forward current due to the exponential term.

Important Observations:

• As the forward voltage increases, the depletion region becomes less effective at blocking charge carriers, and conduction increases.
• The diode operates efficiently in forward bias, allowing current to flow easily, unlike in reverse bias where the current is minimal (leakage current).

Correct Option Analysis:

The correct option accurately describes the behavior of the P-N junction diode under forward bias:

• The space charge region decreases in width because the applied voltage reduces the junction's potential barrier.
• The forward current increases as the applied voltage rises, following the exponential relationship described by the diode equation.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: The width of the space charge region becomes zero, and the forward current increases.

This option is incorrect. The space charge region does not completely disappear under forward bias; it only narrows. While the forward current does increase with rising voltage, the depletion region remains present, albeit reduced in width.

Option 2: The width of the space charge region becomes zero, and the forward current decreases.

Similar to option 1, this option incorrectly assumes the depletion region vanishes under forward bias. Additionally, the claim that the forward current decreases is contrary to the diode's behavior, as the current actually increases with increasing forward voltage.

Option 4: The width of the space charge region increases, and the forward current decreases.

This option is incorrect because, under forward bias, the width of the space charge region decreases, not increases. Moreover, the forward current does not decrease; it increases as the applied voltage rises.

Option 5: This option is not relevant in the context of the given question.

Conclusion:

Understanding the behavior of a P-N junction diode under forward bias is essential for correctly analyzing its operation. The width of the space charge region decreases as the applied voltage reduces the junction's potential barrier, enabling more charge carriers to flow across the junction. Consequently, the forward current increases exponentially with rising voltage, as described by the diode equation. This fundamental principle underpins the operation of diodes in various electronic applications, such as rectification and signal modulation.